1. Field of the Invention
The present invention relates to a scanning device for scanning documents in a multifunctional machine and, more specifically, to a scanning method and apparatus that is capable of enhancing the recorded image resolution along the main scanning axis.
2. Background Art
A scanning device may include many components, such as an optical illumination system, a housing that maintains the document being scanned in a flat position, and a photographic optical system that focuses the light transmitted through, or reflected by, the document onto a reading device. Light from the illumination source is transmitted and optically focused in a line on the source document, and then light from the source document is focused on the reading device. This light is then read as a single line of data. Following the single line of data reading, the scanner moves the source document by a single line increment, changing the position of the original document. Typically, a scanning sensor module `reads` a document using the same space frequency along both the main scanning axis, an axis perpendicular to the paper transport direction, and the sub scanning axis, an axis parallel to the paper transport direction. Thus, the reading accuracy along the sub scanning axis is affected by the pixel pitch, which makes it difficult to improve reading accuracy.
Some developments in scanning devices have been concerned with increasing the resolution of images by varying either the pixel clock or the projection lens used by the scanning device. Some techniques are shown, for example, in U.S. Pat. No. 5,671,069 to Kodama entitled Pixel Clock Generator, and U.S. Pat. No. 5,430,550 to Hashimoto entitled Image Reading Apparatus Which Corrects for Positional and Angular Deviation Between the Image Sensor and the Document. Reading accuracy has also been improved by reading a document with a greater spatial frequency along the sub scanning axis than along the main scanning axis, as detailed in U.S. Pat. No. 5,181,128 to Fukatsu entitled Method and Apparatus for Reading Image Using a Line Image Sensor.
Some developments have been geared towards scanning devices having only one scanning sensor module, as shown, for example, in U.S. Pat. No. 5,646,746 to Tazawa entitled image Input Device Having Carriage That Includes a Movable Origin Detector, that details using an origin sensor to allow a normal sized scanning sensor module to read longer than standard sized media. Another example is shown in U.S. Pat. No. 5,245,447 to Stemmle entitled Indexing Mechanism for Compact Scanner, that mentions a low cost, portable, and single carriage motor scanning device. U.S. Pat. No. 5,084,773 to Yonenaga entitled Image Reading Processing Apparatus, mentions a scanning sensor module that moves in one direction and then reverses to move in another direction to allow the editing of the originally read data.
Speed is an important factor when designing a scanning device. U.S. Pat. No. 5,640,251 to Tone entitled Image Reading Device, shows a reading small sized documents. U.S. Pat. No. 5,687,010 to Van Tilborg entitled Dual Motion Scanning Method and Apparatus Therefor, mentions a technique for performing a pre scan and a main scan that increases the scanning speed.
The use of multiple scanner sensor modules has facilitated improved performance in some scanning devices, albeit at the additional expense of multiple modules and more precise controlling methods. U.S. Pat. No. 4,999,717 to Nagashima entitled Image Reading Apparatus, shows a technique for using multiple line sensors to provide more image data, thus allowing the scanning device to edit out particular data while scanning. U.S. Pat. No. 5,689,347 to Naoi entitled Signal Processing Apparatus, mentions using two scanning sensor modules that are spaced a half line apart that output signals on separate channels to allow for data to be read out faster than it is put into storage without needing to increase the memory of the computer. U.S. Pat. No. 5,570,206 to Yoshinaga entitled Image Reading Apparatus, shows a plurality of scanning sensor modules mounted on a common carriage to increase the amount of information that can be stored without increasing the necessary scanning time.
A scanning device may be constructed using a timing belt for slidably driving a scanning sensor module. In addition, a slide shaft may be used to guide the moving of the scanning sensor module. The slide shaft is rotatably fixed in the frame of the scanning device.
A scanning operation by a scanner device having the above structure is performed along the main scanning axis and along the sub scanning axis. The scan resolution along a direction parallel to the sub scanning axis is dependent on the size of the incremental step that the scanning sensor module is moved by the step motor. On the other hand, the scan resolution obtainable along the main scanning axis is dependant on the particular resolution of the scanning sensor module. That is, if the scanning sensor module only has a resolution of 300 dots per inch, then the resolution of the scanner will not exceed 300 dots per inch along the direction of the main scanning axis. However, the scan resolution can exceed 300 dots per inch along the sub scanning axis.
Even though the maximum resolution in a direction parallel to the main scanning axis is dependant on the resolution of the scanning sensor, the overall resolution of the recorded document can be slightly increased by reducing the incremental movement of the scanning sensor module along the sub scanning axis. For example, when the resolution of the scanning sensor is 300 dots per inch, the amount of movement of the scanning sensor module along the sub scanning axis is 84.7.mu. (1/300 inch). In order to increase the resolution along the sub scanning axis to 600 dots per inch, approximately twice the regular resolution, the movement of the scanning sensor along the sub scanning axis is set to 42.35.mu., approximately half the incremental step normally used. Unfortunately, since the resolution in a direction along the main scanning axis is dependant on the resolution of the scanning sensor, changing the incremental step of the scanning sensor module along the sub scanning axis is of marginal help. Data loss is often incurred in situations where black, or another color, occupies only 30% of a pixel. Since the value that is read by the scanning module is white, there is a data loss of 30%. Thus, even if two pixels were 30% black, the image value read and subsequently output to memory would be 0 (or white). As such, marginal data loss is often incurred.
The contemporary art does not provide a solution to marginal data loss for scanning devices with only one scanning sensor module. As such, I believe that it may be possible to improve on the contemporary art by providing an apparatus and a method for increasing the scanning resolution in the main scanning direction of a scanning device that uses only one scanning sensor module, that approximately doubles the scanning resolution along the main scanning axis, that does not require the use of multiple scanning sensor modules, and that is economical to manufacture.